Subject: Re: [libvirt] [Qemu-devel] Modern CPU models cannot be used with libvirt

Date: Sat, 10 Mar 2012 12:24:47 -0600

On 03/10/2012 09:58 AM, Eduardo Habkost wrote:

On Sat, Mar 10, 2012 at 12:42:46PM +0000, Daniel P. Berrange wrote:

I could have sworn we had this discussion a year ago or so, and had decided
that the default CPU models would be in something like /usr/share/qemu/cpu-x86_64.conf
and loaded regardless of the -nodefconfig setting. /etc/qemu/target-x86_64.conf
would be solely for end user configuration changes, not for QEMU builtin
defaults.
But looking at the code in QEMU, it doesn't seem we ever implemented this ?

Arrrgggh. It seems this was implemented as a patch in RHEL-6 qemu RPMs but,
contrary to our normal RHEL development practice, it was not based on
a cherry-pick of an upstream patch :-(
For sake of reference, I'm attaching the two patches from the RHEL6 source
RPM that do what I'm describing
NB, I'm not neccessarily advocating these patches for upstream. I still
maintain that libvirt should write out a config file containing the
exact CPU model description it desires and specify that with -readconfig.
The end result would be identical from QEMU's POV and it would avoid
playing games with QEMU's config loading code.

I agree that libvirt should just write the config somewhere. The problem
here is to define: 1) what information should be mandatory on that
config data; 2) who should be responsible to test and maintain sane
defaults (and where should they be maintained).
The current cpudef definitions are simply too low-level to require it to
be written from scratch. Lots of testing have to be done to make sure we
have working combinations of CPUID bits defined, so they can be used as
defaults or templates. Not facilitating reuse of those tested
defauls/templates by libvirt is duplication of efforts.
Really, if we expect libvirt to define all the CPU bits from scratch on
a config file, we could as well just expect libvirt to open /dev/kvm
itself and call the all CPUID setup ioctl()s itself. That's how
low-level some of the cpudef bits are.

Let's step back here.

Why are you writing these patches? It's probably not because you have a desire
to say -cpu Westmere when you run QEMU on your laptop. I'd wager to say that no
human has ever done that or that if they had, they did so by accident because
they read documentation and thought they had to.

Humans probably do one of two things: 1) no cpu option or 2) -cpu host.

So then why are you introducing -cpu Westmere? Because ovirt-engine has a
concept of datacenters and the entire datacenter has to use a compatible CPU
model to allow migration compatibility. Today, the interface that ovirt-engine
exposes is based on CPU codenames. Presumably ovirt-engine wants to add a
Westmere CPU group and as such have levied a requirement down the stack to QEMU.

But there's no intrinsic reason why it uses CPU model names. VMware doesn't do
this. It has a concept of compatibility groups[1].

oVirt could just as well define compatibility groups like GroupA, GroupB,
GroupC, etc. and then the -cpu option we would be discussing would be -cpu GroupA.

This is why it's a configuration option and not builtin to QEMU. It's a user
interface as as such, should be defined at a higher level.

Perhaps it really should be VDSM that is providing the model info to libvirt?
Then they can add whatever groups then want whenever they want as long as we
have the appropriate feature bits.

P.S. I spent 30 minutes the other day helping a user who was attempting to
figure out whether his processor was a Conroe, Penryn, etc. Making this
determination is fairly difficult and it makes me wonder whether having CPU code
names is even the best interface for oVirt..

(Also, there are additional low-level bits that really have to be
maintained somewhere, just to have sane defaults. Currently many CPUID
leafs are exposed to the guest without letting the user control them,
and worse: without keeping stability of guest-visible bits when
upgrading Qemu or the host kernel. And that's what machine-types are
for: to have sane defaults to be used as base.)
Let me give you a practical example: I had a bug report about improper
CPU topology information[1]. After investigating it, I have found out
that the "level" cpudef field is too low; CPU core topology information
is provided on CPUID leaf 4, and most of the Intel CPU models on Qemu
have level=2 today (I don't know why). So, Qemu is responsible for
exposing CPU topology information set using '-smp' to the guest OS, but
libvirt would have to be responsible for choosing a proper "level" value
that makes that information visible to the guest. We can _allow_ libvirt
to fiddle with these low-level bits, of course, but requiring every
management layer to build this low-level information from scratch is
just a recipe to waste developer time.
(And I really hope that there's no plan to require all those low-level
bits to appear as-is on the libvirt XML definitions. Because that would
require users to read the Intel 64 and IA-32 Architectures Software
Developer's Manual, or the AMD64 Architecture Programmer's Manual and
BIOS and Kernel Developer's Guides, just to understand why something is
not working on his Virtual Machine.)
[1] https://bugzilla.redhat.com/show_bug.cgi?id=689665